1. Field of the Invention
The present invention relates to an amplifier, a data driver and a display apparatus having the amplifier. More particularly, the present invention relates to an amplifier capable of an offset compensation, a data driver and a display apparatus having the amplifier.
2. Description of the Related Art
Generally, a polysilicon thin film transistor (p-Si TFT) is widely used for a driver circuit of a liquid crystal display apparatus. Recently, the SOG (System On Glass) product including not only a gate driver circuit but also a source driver integrated circuit (IC) that is mounted on glass is under development.
A source follower amplifier is connected to an output terminal of a digital-analog converter (DAC) in the source driver IC. There are various kinds of source follower amplifier. One of the source amplifiers is a compensation type OP-AMP. The compensation type OP-AMP has wide range output, high speed and good linearity.
FIG. 1 is a circuit diagram illustrating a conventional push-pull amplifier. In detail, FIG. 1 is a circuit diagram illustrating a conventional push-pull amplifier having a compensation type OP-AMP.
Referring to FIG. 1, a conventional push-pull amplifier includes a biasing section 10, a first differential amplifying section 20, a second differential amplifying section 30 and an output section 40.
The first biasing section 10 provides first and second bias currents to the first and second differential amplifying sections 20 and 30, respectively, based on first and second power source voltages VDD and VSS.
The first differential amplifying section 20 includes P-channel differential pair having N-channel current mirror. The first differential amplifying section 20 amplifies 1o a difference between an external input voltage and an output voltage based on the first bias current to provide the output section 40 with a first amplified voltage corresponding to the amplified difference.
The second differential amplifying section 30 includes N-channel differential pair having P-channel current mirror. The second differential amplifying section 30 amplifies a difference between an external input voltage and an output voltage based on the second bias current to provide the output section 40 with a second amplified voltage corresponding to the amplified difference.
The output section 40 pulls down the output voltage VOUT to output the second power source voltage VSS based on the first amplified voltage, and the output section 40 pulls up the output voltage VOUT to output the first power source voltage VDD based on the second amplified voltage.
When a threshold voltage of an N-channel transistor is raised, the first and second amplified voltages outputted from the first and second differential amplifying sections 20 and 30, respectively, are raised. Therefore, a linear region is deformed, as the first and second amplified voltages are increased, so that variations between inputs and outputs occur. For the same reason, a change in a threshold voltage of a P-channel transistor influences the outputs.
Therefore, when the OP-AMP employs the p-Si TFT, a large variation of the output occurs even when same input voltage is applied to the OP-AMP. The variation of the output is referred to as “offset” or “offset voltage”. The offset shows large variance in accordance with the characteristics of transistors.
The liquid crystal display panel includes a plurality of p-SI TFTs, and the p-Si TFTs show large variance in threshold voltage Vth over the panels, even within a panel. Therefore, when the variation of the threshold voltage is not compensated, different outputs are outputted in accordance with the pixels of the liquid crystal display panel, even though same data voltage is applied to the p-Si TFTs of the liquid crystal display panel. Therefore, display quality is lowered.